Role of hydrogen and deuterium on theV−-center formation in MgO

The formation of the $V^{-}$ center in MgO by ionizing radiation is found to be contingent on the presence of an impurity: hydrogen. The attainable $V^{-}$ concentration is dependent upon the initial $V_{\mathrm{OH}}^{-}$ concentration detected by the 3296-${\mathrm{cm}}^{-1\mathrm{}}$ infrared absorption band. A similar dependence on the $V_{\mathrm{OD}}^{-}$ concentration (2445-${\mathrm{cm}}^{-1\mathrm{}}$ band) in deuterated crystals is also observed. Photographs illustrate that the $V^{-}$ density distribution for a given crystal is identical to that for the $V_{\mathrm{OH}}$ center in the same crystal. Quenching crystals, as compared with slow cooling, leads to larger $V_{\mathrm{OH}}^{-}$ or $V_{\mathrm{OD}}^{-}$ bands and, upon intense electron irradiation, results in larger $V^{-}$ concentrations. As the $V^{-}$ concentration is enhanced by electron irradiation, the $V_{\mathrm{OH}}$ and $V_{\mathrm{OH}}^{-}$ concentrations diminish and ultimately vanish at doses of ∼ 1 × ${10}^{18}$ e/${\mathrm{cm}}^2$. Hydrogen-free (or deuterium-free) crystals are immune to $V^{-}$-center formation, regardless of thermal treatment. The experimental evidences can be understood on the basis of a formation mechanism which involves the displacement of hydrogen from the $V_{\mathrm{OH}}^{-}$ site by ionizing radiation. The enhancement of the infrared absorption bands at 3700 and 2723 ${\mathrm{cm}}^{-1\mathrm{}}$ in certain crystals due to electron irradiation indicates that some of the displaced hydrogen and deuterium ions form Mg${\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_2$ and Mg${\mathrm{}\left(\mathrm{OD}\right)\mathrm{}}_2$ precipitates, respectively.